1. Field of the Invention
The invention relates to arrangements for actuating an element in a microlithographic projection exposure apparatus.
2. Prior Art
Microlithography is used for producing microstructured components such as, for example, integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus having an illumination device and a projection lens. The image of a mask (=reticle) illuminated via the illumination device is in this case projected via the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In a projection exposure apparatus designed for EUV (i.e. for electromagnetic radiation having a wavelength of less than 15 nm), for lack of light-transmissive materials being available, mirrors are used as optical components for the imaging process. The mirrors can be fixed on a carrying frame and can be designed to be at least partly manipulatable in order to enable the respective mirror to be moved for example in six degrees of freedom (i.e. with regard to displacements in the three spatial directions x, y and z and with regard to rotations Rx, Ry and Rz about the corresponding axes). In this case, the position of the mirrors can be determined via position sensors fixed to a sensor frame.
In a projection exposure apparatus designed for EUV, mirrors designed to be manipulatable are used both as actively deformable mirrors, in the case of which changes in the optical properties that occur e.g. during the operation of the projection exposure apparatus and resultant imaging aberrations, e.g. on account of thermal influences, can be compensated for by active deformation, and as non-actively deformable mirrors, in the case of which no targeted deformation is effected.
The positional control of such mirrors serves, in conjunction with a suitable actuator system (e.g. with Lorentz actuators), to keep the mirrors in their position as stably as possible, such that a deviation of the mirror positions that is measured via the position sensors is as small as possible. One approach that is possible in principle for this purpose consists in increasing the controller gain and thus increasing the control bandwidth. In this case, however, the problem occurs in practice that the mirrors are not ideally rigid bodies, but rather each have specific natural frequencies of the mechanical structures (e.g. of a typical order of magnitude in the range of 2-3 kHz), wherein the corresponding natural frequency spectra for the dimensions of the mirrors and of the carrying and measuring structures, the dimensions increasing with increasing numerical apertures, are shifted further and further toward lower frequencies. This applies all the more to actively deformable mirrors, which have to be designed to be deformable and thus compliant in a targeted manner. An excitation of the natural frequencies via the actuators can have the effect, however, that on account of the relatively low damping in the control loop comparatively large amplitudes are detected by the respective position sensors, as a result of which the stability of the control loop can be jeopardized and active positional control can no longer be operated stably or can be operated only with low control quality.
With regard to the prior art, reference is made for example to U.S. Pat. No. 6,842,277 B2, US 2007/0284502 A1 and the publication “Benefits of over-actuation in motion systems”, by M. G. E. Schneiders et al., Proceedings of the 2004 American Control Conference (ACC 2004), Boston (2004).